PROJECT SUMMARY/ABSTRACT OF SUPPLEMENT This project will advance understanding of the biological mechanisms that underlie different paths in cognitive aging, cognitive decline and Alzheimer disease (AD) and AD-related dementias (ADRD) through examination of DNA methylation (DNAm) ? a marker that can reflect epigenetic age and gene expression ? and its relation to neuropsychiatric symptoms (NPS) among those who may be in preclinical phases of AD or have mild cognitive impairment (MCI). This project features novel technology ? the latest-generation Illumina MethylationEPIC BeadChip 850K platform ? and is embedded within the randomized study design of VITAL (VITamin D and OmegA-3 TriaL; NCT01169259) and VITAL-DEP (NCT01696435), a late-life depression prevention ancillary study. The proposal is directly relevant to two NIA high-priority topics: 1) ?geroscience approaches to AD? and 2) understanding ?molecular mechanisms of NPS in ADRD? using ?high-dimensional molecular and phenotypic data from aging cohorts?. The conceptual framework of this proposal is that: accelerated biological aging predisposes to worse brain aging; depression, anxiety and other psychopathology are involved in faster aging of both body and brain; furthermore, cognitive impairment accompanied by NPS vs. without NPS may represent underlying mechanisms that generate higher risk for worse AD/ADRD outcomes. Genome-wide methylation analysis is a powerful approach to address this model. First, pilot data in VITAL- DEP, and results from other groups, indicate that DNAm age correlates more strongly with aging than other markers. Second, DNAm age is a sensitive indicator of change in molecular aging over the relevant follow-up period for observing cognitive decline. Third, DNA methylation is a biomarker not only of aging but also of gene expression. Indeed, a DNAm analytic approach to investigate differences in epigenetic aging and gene expression may uncover targetable molecular mechanisms that undergird varying cognitive and NPS phenotypes and cognitive outcomes. We propose three Specific Aims to test hypotheses in our model. First, we will determine differences in epigenetic aging by contrasting cognitive and NPS phenotypes: we will assess baseline differences in epigenetic aging among VITAL-DEP participants (aged 50-90 years) with normal cognition, MCI without NPS and MCI with NPS, and test whether NPS severity correlates with epigenetic aging. Second, we will evaluate genome-wide differences in methylation (i.e., at specific CpG sites) in MCI with vs. without NPS. Third, in a longitudinal analysis using repeated measures of both DNA methylation and cognitive tests, we will determine whether epigenetic aging correlates with cognitive decline; we will also explore interactions with NPS and the vitamin D and omega-3 randomized agents. Strong DNAm pilot data in VITAL-DEP and the ability to leverage this large, well-characterized cohort and its biospecimen resources and infrastructure provide reassurance that this project can achieve its novel, high-impact aims.